Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a metastable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.

Research in topological matter has expanded to include the Dirac and Weyl semimetals 1-10 , which feature three-dimensional Dirac states protected by symmetry. Zirconium pentatelluride has been of recent interest as a potential Dirac or Weyl semimetal material. Here, we report the results of experiments performed by in situ three-dimensional doubleaxis rotation to extract the full 4π solid angular dependence of the transport properties. A clear anomalous Hall effect is detected in every sample studied, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Large anomalous Hall signals develop when the magnetic field is rotated in the plane of the stacked quasi-two-dimensional layers, with the values vanishing above about 60 K, where the negative longitudinal magnetoresistance also disappears. This suggests a close relation in their origins, which we attribute to the Berry curvature generated by the Weyl nodes. Zirconium pentatelluride (ZrTe 5) has recently attracted considerable attention, following the observation of negative longitudinal magnetoresistance (LMR) 11. This negative LMR has been identified with the chiral anomaly 12-14 that is predicted to occur in Dirac and Weyl semimetals 1-10 and was recently observed in Na 3 Bi and GdPtBi 15,16. However, despite the observation of the negative LMR, there are no theoretical predictions showing that ZrTe 5 is a threedimensional (3D) Dirac or Weyl semimetal, in contrast to both Na 3 Bi (ref. 17) and Cd 3 As 2 (ref. 18). Furthermore, the results of angleresolved photoemission spectroscopy (ARPES) experiments 11,19-23 are not yet conclusive. It is therefore of interest to investigate other unusual transport properties of ZrTe 5 , especially the Hall response engendered by the Berry curvature. For Dirac and Weyl semimetals in an electric field E, a finite Berry curvature leads to an anomalous velocity Ω = × v E

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi2Se3 by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:Sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.PACS numbers: 79.60.Bm, 78.47.JThree dimensional topological insulators (TIs) are materials characterized by an insulating bulk and a conductive surface electronic structure. The hallmark of a TI is its linearly dispersing surface state (SS) which is guaranteed to cross the band gap separating the bulk valence band (VB) and conduction band (CB) [1][2][3][4]. In addition to this so-called topological protection, the SS has a chiral spin texture [5,6] which offers the electrons protection against backscattering and has great appeal for spintronics applications [7,8]. A number of interesting phenomena are associated with optical coupling to TIs, such as colossal Kerr rotation [9], divergent photon absorption [10], spin transport [11,12], and a long-lived SS population [13]. To exploit these phenomena, a detailed understanding of the response to optical excitation is required. A number of studies have investigated electron and phonon dynamics initiated by ultrafast optical excitation either by optical reflectivity [14,15], second harmonic generation [16], or time-and angle-resolved photoemission spectroscopy (trARPES) [13,[17][18][19]. However, these studies have not explicitly addressed the electronic transition driven by the excitation. A detailed understanding of the unoccupied electronic structure is required to understand precisely how photons couple to electrons in these materials. Inverse photoemission has previously been applied to Bi 2 Se 3 , but the energy resolution of 0.56 eV and lack of momentum information precluded the ability to get a detailed picture of the unoccupied states [20]. More recently, significant progress was made by Niesner et. al., who used two-photon photoemission spectroscopy to investigate the material family Bi 2 Te x Se 3−x [21]. Remarkably, they identified signatures of a second SS in the unoccupied states which is expected to have the same topological protection and chiral spin texture [21,22].In this work, we use a combination of one-photon photoemission (1PPE) and two-photon photoemission (2PPE) to resolve the occupied and unoccupied electronic structure of p− and n−type Bi 2 Se 3 , which is substantiated with theoretical band structure calculations. The energy and momentum resolution is drastically improved over previous work [21], allowing an unambiguous identification of the second SS. Moreover, we show that in n-type samples, 1.5 eV photo...

The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a "coherent lock-in" measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.

We report time-and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. DFT calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.PACS numbers: 78.47. J-, 73.20.-r, 63.20.kd, 79.60.-i Topological insulators (TIs) are materials that behave as electronic insulators in their bulks, but have robust surface states (SSs) which enable metallic conduction [1][2][3][4][5]. A particularly exciting property of the SS is that it is strongly spin-polarized, with the electrons' spinorientations locked perpendicular to their momenta [6,7]. While this novel spin texture greatly reduces the phase space for spin-conserving scattering events [8,9], there still remain scattering processes which give the SS electrons a finite lifetime and limit their ballistic transport, and thus must be considered for device applications [10].Among these scattering processes, those driven by electron-phonon coupling (EPC) in particular have been the subject of intense study because they affect any finitetemperature application of the TIs. The fundamental questions regarding EPC are: Which electronic states are involved, and to which phonon modes do they couple? A number of recent measurements including helium atom scattering [11][12][13] and inelastic transport [14] seem to be arriving at a consensus that scattering in Bi 2 Se 3 is dominated by a ∼ 7 − 8 meV optical A 1g phonon mode [15]. However, because of the coexistence of bulk and surface carriers in Bi 2 Se 3 [16], it is not clear from these experiments whether the measured A 1g mode coupling corresponds to EPC in the bulk or surface states. In principle, this could be investigated by angle-resolved photoemission spectroscopy (ARPES) because of its capability to measure the electron self-energy directly on the SS band. However, the ARPES results reported in the literature have been scattered: two works identified no particular mode coupling to the SS [17,18], one identified an ∼ 18 meV mode [19], while yet another identified modes at both ∼3 and ∼18 meV [20]. To-date no measurement has directly observed the A 1g mode coupling to the SS band.The discrepancy in ARPES measurements is likely attributed to the fact that the coupling occurs on a small energy scale accompanied by a weak spectral signature, so very high energy resolution is required to unambiguously detect it [13]. Here we circumvent this experimental difficul...

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10

There is a great deal of fundamental and practical interest in the possibility of inducing superconductivity in a monolayer of graphene. But while bulk graphite can be made to superconduct when certain metal atoms are intercalated between its graphene sheets, the same has not been achieved in a single layer. Moreover, there is a considerable debate about the precise mechanism of superconductivity in intercalated graphite. Here we report angle-resolved photoelectron spectroscopy measurements of the superconducting graphite intercalation compound CaC6 that distinctly resolve both its intercalant-derived interlayer band and its graphene-derived π* band. Our results indicate the opening of a superconducting gap in the π* band and reveal a substantial contribution to the total electron–phonon-coupling strength from the π*-interlayer interband interaction. Combined with theoretical predictions, these results provide a complete account for the superconducting mechanism in graphite intercalation compounds and lend support to the idea of realizing superconducting graphene by creating an adatom superlattice.